The invention is an improvement and an innovation on the basis of previous work. In the field, the present research institute has already obtained two patents in China, namely Single-Photon Counting Imaging System and Method (application number or patent number: 201110103559.3, applicant or patentee: Center for Space Science and Applied Research, Chinese Academy of Sciences) and ULTRA-WEAK LIGHT MULTISPECTRAL IMAGING METHOD AND SYSTEM (application number or patent number: 201110166471.6, applicant or patentee: Center for Space Science and Applied Research, Chinese Academy of Sciences). Both of these two patents were based on the compressed sensing theory, the spatial light modulation technology and the single-photon detection technology and are used for single-photon counting imaging; and the difference between them lies in that the former realizes 2D imaging of an ultra-weak light object on single-photon level only by using a single-photon detector as a detection element, and the latter realizes ultra-weak light multispectral color imaging on single-photon level by using a single-photon counter linear array serving as a detection element together with a spectrophotometer. However, the two patents have the common defects: only static objects can be imaged but without light intensity analysis, time-resolved and spatial 3D resolution capacities; only algorithm simulation is performed without considering the influence of system noise, so the actual reconstruction precision is low; the problem of synchronism of a counting system and a digital mirror device (DMD) is not solved. Therefore, there still exist technical defects in these two patents. In order to solve the problems of time resolution and multi-D parameter detection and imaging, the present invention provides a time-resolved single-photon or ultra-weak light multi-D spectral imaging system and a time-resolved single-photon or ultra-weak light multi-D spectral imaging method for overcoming the above-mentioned drawbacks.
The so-called “time resolution” means to distinguish an interval on the dimension of time, and “ultrahigh time resolution” means to observe physical and chemical transient processes with the capacity of resolving the time of the processes. In a liquid phase, many physical and chemical processes such as cis-trans isomerism and orientation relaxation of molecules, transfer of charges and protons, collision pre-dissociation, energy transfer and fluorescence lifetime of excited molecules, and solvation of electrons in water can be completed in only 10−8 seconds, and it is possible to observe these extremely fast processes timely only through an analysis instrument with a time resolution precision of picoseconds. In the present invention, single-photon detection and counting sampling are expected to be carried out in large-scale or extremely short time intervals.
Moreover, currently the technologies for transient ultra-weak light (such as fluorescence lifetime) measurement mainly comprise the single-molecule detection technology, the time resolution technology and the super-resolution measurement technology, wherein: (1) the single-molecule detection technology mainly comprises wide field confocal fluorescence microscopy, scanning near field optical microscopy (SNOM), total internal reflection fluorescence (TIRF) microscopy, atomic force optical microscopy (AFOM) and Raman scattering technology; (2) the time resolution technology mainly comprises fluorescence lifetime imaging microscopy (FLIM), double-photon fluorescence lifetime microscopic imaging, fluorescence lifetime correlation spectroscopy (FCS) and multi-D fluorescence lifetime microscopy; and (3) the super-resolution measurement technology mainly comprises stimulated emission depletion (STED) microscopy, position sensitive microscopy (PALM, STORM, dSTORM and GSDIM), optical fluctuation (SOFI) microscopy and fluorescence resonance energy transfer (FRET) microscopy. According to a fluorescence lifetime imaging and related spectral quantitative measurement method for biological macromolecules, single-point fluorescence lifetime and related spectral measurement is carried out by an FLIM or FCS system, and then fluorescence lifetime imaging and related spectral quantitative measurement of the biological macromolecules is performed by adopting a laser beam scanning or sample scanning system. Because of the poor stability of nano displacement scanning platform and the complexity of the scanning process, not only is the manufacturing cost increased, but also the test time for nano materials and biological macromolecules is greatly prolonged and so the success rate is also significantly affected. According to a high-resolution microstructure imaging measurement method for a nano material, graphic representation is carried out generally by adopting an electron scanning microscope. Because the sample under measurement may be damaged by ionization of high-energy electrons, it is impossible to realize non-destructive imaging measurement of the bioactive molecules and the nano materials. The above technologies have such a common problem that spatial fluorescence lifetime measurement and related spectral analysis of an observed object cannot be carried out at the same time. With the development of scientific research towards high spatial resolution, high time resolution, multiband, fast detection, photon excitation and the like, it increasingly seems that the functions cannot meet the increasing requirements any more.
According to the global optical microscopy market analysis reports of Future Markets, Inc. in August 2011 and Global Industry Analysts, Inc. in February 2011, the annual profit of the global optical microscopy market is predicted to exceed 4.1 billion dollars until 2017. The demand for advanced, high-resolution and high-precision micro-imaging in current and future new markets is continually increased, and particularly the technical progresses in the fields of semiconductors, electronics, industry, micro electromechanical systems, biomedicine pharmacy, nano technology and nano material research and development greatly promote the increase of demand of related instruments. In brief, the time-resolved imaging spectrometry has attractive market development prospect, and can greatly promote the development of related industries. Basic scientific research of nano materials, crystal materials, nondestructive detection of the materials, quantum dot systems, photonic crystal, high-speed phenomenon detection, high-resolution spectrum measurement and quantum chemical basic science research in physical and chemical research as well as forefront problems of biophysical science of medical diagnosis, monomolecular biophysics, nano biological effect, molecular bionics, brain functions and recognition, proteomics and the like in biophysical research need the novel imaging system and method to realize the breakthrough of multi-disciplinary, multi-parameter and multi-scale quantitative research. A lot of research about life science, material science, chemistry and energy science indispensably needs to know components included in the investigated objects and changes of the components therein, and spectral imaging analysis is one of the most effective nondestructive analysis means for component analysis; the research requirement for where the components of the investigated object are distributed and how the components have changed, namely spatial distribution information of the changed components, can be realized by means of the spectral imaging analysis; with the development of scientific research, people not only need to know the spatial distribution information of the components, but also need to know the changing process of the components with time, particularly whether any intermediate product(s) being produced, the process of forming, annihilation and the acting mechanism of the intermediate products in the component change process, thus raising a requirement of time resolution for imaging spectrometry; and in order to achieve the following targets of determining the components, the quantity of each components, time resolved and location-determined of the components, two critical technical obstacles, namely the problems of dimensions and sensitivity, cannot be avoided no matter which solution is adopted. In the existing imaging spectrometry, the spectral imaging analysis of 1D spectrum or any dimension of spectrum of 2D planar images must be realized in an auxiliary scanning mode. This working mode brings the defects that the time duration for sampling must reserve sufficient time for scanning, and moreover, it is difficult to realize ultrahigh time resolution in the sense of electronics or the devices themselves because a planar sensor is composed of a large number of photosensitive detection units. Internationally, currently implemented solutions of instruments having time-resolved imaging spectrometry function can be divided into two types: scheme I, the function of 2D spatial scanning is added to a time-resolved spectrometer, but this scheme has the defects that its efficiency is low due to the time conflict between time resolution and spatial scanning and so its whole performance cannot meet higher scientific research requirements; and scheme II, the function of time resolution is added to an image spectrometer or an optical channel is added to a single-photon micro-imaging device, but this solution has the defects that its time resolution capability is difficult to improve and cannot meet the time requirement of time-resolved spectrum and so its application range is limited.
From the view of the analysis of optical signal intensity of a research object, introducing spatial resolution into the function of a spectrometer means the intensity of source signals which can be captured by an optical sensor is reduced, and further the requirement of time-resolved measurement means further reduction of the intensity of source signals. The lower the intensity of the source signal is, the lower the intensity of the fluorescence signals to be captured by a detector. The sensitivity of detectors is the key performance of the time-resolved imaging spectrometer, i.e., the higher the sensitivity of the detector is, the better the performance of the equipment. Single-photon detectors can detect the minimum optical energy, which is the detection limit of light, hence single-photon detection technology is the final pursuit of the time-resolved imaging spectrometer ever since.
Because of the above-mentioned restriction of principle and detection technologies, a time-resolved single-photon multi-D imaging system can be possibly realized in real sense only by new principles and new methods. The present invention realizes measurements with 2D spatial resolution by adopting the latest developed compressed sensing theory and combining modern mature technical conditions through a single-photon point detector, wherein one dimension of a 2D spatial distribution will be saved, and compared with a surface element detector, the single-photon point detector has more advantages in the aspects of detection sensitivity and wavelength range, that is to say, with the extremely high sensitivity, high flux measurement, high signal-to-noise ratio, wide wavelength range and low cost, single-photon counting imaging realized by using a point detector will certainly become an important development trend of future single-photon level imaging. Single-photon counting imaging is an ultra-weak light detection technology, wherein the light intensity distribution is represented by recording the counts of the photons and the probability of detection of the photons to reconstruct an image.
A linear array or array, Geiger mode single-photon detector can be used to realize imaging spectrometry, with a time resolution precision of picosecond, a spatial resolution of nano level and a detection sensitivity of single-photon detection level, so as to realize in real sense single-photon time-resolved imaging spectrometry, which is in nature different from traditional imaging spectrometry. Here, the imaging spectrometry is an important technology for acquiring and displaying precise spectral information, because a spectral image contains more information about the spectrum, and the multispectral imaging technology greatly overcomes the phenomenon of metamerism. Moreover, the single photon, as an ultra-weak light, is regarded as the minimum energy unit of light that cannot be divided further. The discrete photon pulse signal is the detectable limit, and generally detected by a single-photon detector. When a counting-type single-photon detector works in a saturation state, whose sensitivity being in a single-photon level, it obtains a photon density image by adopting a statistical method; and when the detector works in a sub-saturation state, then the amplitude of the electrical signals output by the detector varies with the changes of the number of detected photons, and so an ultra-weak light image is acquired via these electrical signals. Ultra-weak natural discrete signals are identified and extracted by adopting pulse discriminating technology and digital counting technology in the single-photon counting method, wherein the performance of the method is affected very little by those instable factors and is free of most influence of the thermal noise of the detectors, so the digital signal output through the method has a greatly improved signal-to-noise ratio.
Since the theoretical basis of the invention is the compressed sensing theory and the spatial light modulator technology, the compressed sensing theory and the spatial light modulator technology of the prior art will be described in details hereafter.
The compressed sensing principle is a brand-new mathematical theory proposed by Donoho, Tao, Candès et al., which can perfectly recover original signals in a randomly sampling mode with smaller number of the sampling (far fewer than the limit of Nyquist/Shannon sampling theorem) and has higher robustness. The principle is mainly divided into three steps: compressive sampling, sparse transform and algorithm reconstruction, wherein the compressive sampling is a process for mapping the signals under measurement from high-D signals to low-D ones; the sparse transform means to select a proper Ψ, so that x′ obtained after the Ψ transform of x is sparse, namely x can be sparsely represented under the Ψ framework; and the algorithm reconstruction is a process for solving y=AΨx′+e under the condition that the observation data y, the measurement matrix A and the framework Ψ are known, and finally, x is recovered according to x=Σi=1Nx′iψi.
The spatial light modulator (SLM for short) is a device capable of loading information to a 1D or 2D optical data field and performing optical information processing, to be more specific, it can change the amplitude or the intensity, the phase, the polarization state and the wavelength of the spatial light distribution or convert the incoherent light into the coherent light, under the control of electric driving signals or other signals changed with time. The most typical representative of the SLM is a digital micro-mirror device (DMD for short), which is the most precise optical switch in the world. The core of the DMD is a micro-mirror array (mainstream DMD consists of a 1,024×768 array, maximally up to 2,048×1,152) consisting of thousands of micro-mirrors arranged on hinges, of size 14 μm×14 μm (or 16 μm×16 μm) for each mirror and being able to switch on/off the light on one pixel. Each of the micro-mirrors is suspended and can be caused to incline to its two sides by about 10 to 12° (+12° and −12° are selected here) in an electrostatic form by performing electronic addressing to a storage unit under each micro-mirror with a binary array signal, and the two states being marked as 1 (corresponding to “on”) and 0 (corresponding to “off”), respectively; and when the micro-mirrors do not work, they are in an “anchorage” state.